.btn{
    width: 100%;
    height: 40px;
    position: fixed;
    bottom: 0;
    left: 0;
    z-index: 999;
    background: #fff;
}
.btn button{
    width: 100px;
    height: 30px;
    background: blue;
    border: none;
    color: #fff;
    margin: 5px auto;
    display: block;
    border-radius: 5px;
}